The field of the invention relates to a belt used for extracting water from a web of material, and more particularly from a fibrous web formed in a papermaking machine.
During the papermaking process, a web is formed by depositing a fibrous slurry on a forming wire. A large amount of water is drained from the slurry during this process, after which the newly-formed web proceeds to a press section. The press section includes a series of press nips. The web finally proceeds to a drying section including heated dryer drums where the water content is reduced to a desirable level.
In view of the high cost of energy, it is desirable to remove as much water as possible from the web prior to its entering the drying section. The dryer drums in this section are often heated by steam and costs can be substantial if a large amount of water needs to be removed.
The use of the extended nip press has been found to be advantageous over the use of nips formed by pairs of adjacent rollers. By extending the time the web is subjected to pressure in the nip, a greater amount of water can be removed. This fact has been recognized by those skilled in the art, and several patents have been granted in the area. These patents include Re. U.S. Pat. Nos. 30,268, 4,201,624, 4,229,253 and 4,229,254.
Fibrous webs dewatered by a conventional belt in the extended nip press have been heavy weight flow controlled, sheets such as linerboard which has a basis weight of between 110 to 410 grams/sq. meter. The ability to dewater in "flow controlled" sheets is time dependent. The longer the dwell time in the press nip, the more water can be removed. The prior art sandwich felt system of dewatering linerboard in the extended nip press requires that the top roll be vented. Venting is generally accomplished by machining annular grooves in the top roll 0.100" deep by 0.02" wide at 8 grooves to the inch of width. Water from the fibrous web is squeezed and passes into and through the sandwiched felts and channels into these grooves. Without the grooves, less water could be removed in the extended nip.
Another consideration is that with the dewatering of lightweight webs of paper such as newsprint or fine paper which range from 40 to 95 grams/sq. meter, the function is more pressure controlled than flow controlled. Because lightweight paper dewatering is substantially pressure controlled, a grooved top roll would mark the sheet via the high loading in the extended nip press. The configuration of the extended nip press for use with fine paper or newsprint sheets would require the use of a smooth hard surface top roll and the elimination of the top sandwich felt which would provide the uniform pressure and impart a smooth surface to the web to be dewatered.
The problem with this configuration is that there is no means for the expelled water to be evacuated from the extended nip with the grooves have been eliminated from the top roll.
Attempts to make belts that have grooves on the felt side have been made. Unfortunately, the grooves mark the sheet and only provide a minimum of 0.016 cubic inches of void per square inch of surface area.
In using the extended press to dewater a fibrous web, the web has typically been sandwiched between two moisture-absorbing felts and a belt. The felts are trained around a cylindrical press roll with the web between them while the belt is arranged for applying pressure to the felts and roll. A pressure shoe exerts pressure on the belt in the press area and the shoe and belt are lubricated with hydraulic oil.